1. Field of the Invention
The present invention relates to an optical element mounted body, and an optical semiconductor module using the optical element mounted body.
2. Related Art
Optical semiconductor elements of various kinds are used in optical modules in which an optical component, such as an optical fiber or an optical waveguide, is optically coupled to a light-emitting end face or a light-receiving end face of the optical semiconductor element. In the optical modules, the optical axis of the optical semiconductor element and that of the optical component have to be aligned each other.
In particular, in the case of optically coupling an optical fiber to an optical semiconductor element such as a semiconductor laser element (LD) or a waveguide-type photodiode (WG-PD), the alignment precision must be controlled within +/−1 μm or so.
In the related art, the alignment has been exemplarily done in a manner as follows: facing an end face of an optical fiber to a light-emitting end face of an LD when driving the LD, monitoring the output light coupled to the optical fiber while searching for a fiber position to attain a maximum light output, until fixing their positions.
The alignment of the above method, however, necessitates a very complicated work to be done, and is not suitable in pursuing cost cutting and mass production of optical modules.
It is for that reason that recently, a method so-called passive alignment is being developed for practical use. In the passive alignment technique, the relative position between an optical semiconductor element and an optical element mounting substrate (hereinafter referred to simply as ‘mounting substrate’), and the relative position between the mounting substrate and an optical component as well, are precisely determined, whereby the optical semiconductor element and the optical component can be aligned through the mounting substrate and optically coupled each other, thus eliminating the need to drive the optical semiconductor element while the alignment.
To be more concrete, in the technique, alignment markers are provided precisely on both the optical semiconductor element and the mounting substrate, which will be used when mounting the optical semiconductor element on the mounting substrate to determine the relative position of the optical semiconductor element and the mounting substrate such that the alignment marker on one is at a predetermined position relative to the alignment marker on the other.
The optical semiconductor element having an alignment marker is known in U.S. Pat. No. 5,715,267, the entire content of which is incorporated herein by reference. FIG. 7 shows a semiconductor laser element disclosed in the U.S. Pat. No. 5,715,267, in which two stripes of mesa are formed in parallel to each other on a substrate 5e with a predetermined distance interposed therebetween in the widthwise direction of the substrate 5e, one of the mesa stripes including an active layer 5c emitting a laser beam while the other having a V-groove marker 5b formed on top of it. Since these mesa stripes are formed simultaneously in one etching process in the course of manufacturing the semiconductor laser element 5, the relative position of the two mesa stripes are precisely fixed, and the separation between the V-groove marker 5b and the active layer 5c as well. The V-groove marker 5b are formed by leaving a dielectric layer on the top of one mesa stripe, which restrains the crystal growth of semiconductor layers thereon when burying the other mesa stripe with the semiconductor material.
In general, optical semiconductor elements are mounted junction-down on mounting substrates, in which the top surface (i.e. the surface formed through crystal growth, or the upper surface 5a in FIG. 7) is faced to the mounting substrate.
This is because the top surface of the optical semiconductor element, which was formed through vapor phase epitaxial growth technique, is superior in the preciseness of thickness of each semiconductor layer, typically being controlled within +/−0.1 μm or so, to the bottom surface which generally has a roughness in the range of +/−10 μm or so even after a polishing process. Hence, the top surface is more suitable than the bottom surface to be used as a reference plane above which the height of the active layer, or a light-emitting/light-receiving portion of the optical semiconductor element, is determined, helping to ensure the alignment precision in the direction perpendicular to a mounting surface of the mounting substrate.
The mounting substrate having an alignment marker is known in U.S. Pat. No. 6,270,263, the entire content of which is incorporated herein by reference. FIG. 9 is a sectional view of the optical module disclosed in U.S. Pat. No. 6,270,263. In the optical module, the mounting substrate 4′ is formed of silicon for example, and has a predetermined wiring patterns and V-grooves formed in parallel to the direction in which optical signals are inputted or outputted through an optical fiber 3, on the surface on which an semiconductor laser element 5 is disposed. The V-grooves on the mounting substrate 4′ serve as alignment markers when mounting the above-described semiconductor laser element 5 having an alignment marker on the mounting substrate 4′.
Using such mounting substrate 4′, the position of the optical fiber 3 relative to the mounting substrate 4′ is determined by engaging the V-grooves on the mounting substrate 4′ with the ridges 2a3 formed precisely on the package 2 with a predetermined positional relation to the longitudinal hole 2a1 holding the optical fiber 3.
Thus, the optical fiber 3 held in the longitudinal hole 2a1 of the package 2 and the semiconductor laser element 5 fixedly positioned on the mounting substrate 4′ using the alignment markers are precisely aligned and optically coupled to each other.
In some cases, the semiconductor laser element 5 as shown in FIG. 7 may have a projection 5d of irregular height generated in the vicinity of and along the V-groove marker 5b (i.e. in the region marked IV in FIG. 7), as shown in FIG. 8. Such projections 5d may be irregularly generated on account of an abnormal crystal growth of semiconductor occurred in the region in a manufacturing process of the semiconductor laser element 5.
In another case, the semiconductor laser element 5 may have a projection 5d of irregular height generated on the edge portion of the upper electrode 5k (i.e. in the region marked V in FIG. 7), as shown in FIG. 10. Such projections 5d may be irregularly generated in an electrode forming process of the semiconductor laser element 5.
When mounting such a semiconductor laser element 5 junction-down on a mounting substrate, the projection 5d can be a cause of gap between the surface of the semiconductor laser element 5 and the mounting surface of the mounting substrate 4′.
Furthermore, in the case where a semiconductor laser element 5′ as shown in FIG. 11(A) is aligned with respect to the mounting substrate by employing an edge of the electrode 5k on upper surface as an alignment marker instead of the V-groove 5b of FIG. 7, an irregular projection on the edge portion of the electrode 5k, that may sometimes appear in the region marked VI in FIG. 11(A) as shown in FIG. 11(B)), could be a cause of gap between the surface of the semiconductor laser element 5′ and the mounting surface of the mounting substrate 4′.
In some cases, such a gap could impede a dissipation of heat generated at the semiconductor laser element to the mounting substrate, deteriorating the performance of the semiconductor laser element.
In another case, such a gap could be a cause of the position of the semiconductor laser element relative to the mounting substrate being different from element to element, causing thereby an aligned position of the optical fiber relative to the semiconductor laser element being different from element to element. Thus, the optical coupling efficiency between the semiconductor laser element and the optical fiber could be different from element to element.
In still another case, such a gap could be a cause of insufficient fixing strength of the semiconductor laser element to the mounting substrate, accounting for the semiconductor laser element being coming off the mounting substrate 4′ when wire-bonding the semiconductor laser element to an external circuit.